Series endings are usually a time for beloved characters to return, and the final episode of The 100 delivered on this promise, but in an unexpected way, because some characters did indeed return, but they weren’t exactly themselves.
Clarke having to determine the fate of humanity, so the judge took the form of Lexa (Alycia Debnam-Carey). Judge-as-Lexa also returned at the end of The 100 episode, on Earth, to tell Clarke something shocking.
Callie re-judges Cadogan in The 100, and thus humanity, during the test. Except she’s not really Callie. As the alien evidence manager (and judge) told Cadogan, Callie is the one she chose to see.
Even though Callie had only appeared on The 100 once before, it was interesting to see Cadogan think he was interacting with her after centuries of wanting to see her again.
Killed on The 100 season 6 when she was used to hosting Simone Lightbourne (Tattiawna Jones and Chelah Horsdal). Abby was also close to Raven Reyes (Lindsey Morgan), which is why she makes a cameo in the finale.
When Clarke failed the test, thus condemning humanity. So, Raven tried to retake the human test and ended up in the Ark with none other than a judge with the face of Abby from The 100.